A range of efficacies have been reported for biomedical HIV prevention interventions, including antiretroviral treatment, male circumcision, preexposure prophylaxis, microbicides, and preventive vaccines. This range of efficacies probably results from the influence of multiple inputs and processes during trials, including the strength and target of the intervention, host factors, target population characteristics, level of HIV exposure, and intervention dose. Expertise in social and behavioral sciences, in conjunction with basic science, clinical research, epidemiology, biostatistics, and community, is needed to understand the influence of these inputs and processes on intervention efficacy, improve trial design and implementation, and enable interpretation of trial results. In particular, social and behavioral sciences provide the means for investigating and identifying populations suitable for recruitment into and retention in trials and for developing and improving measures of HIV exposure and intervention dose, all within the larger sociocultural context. Integration of social and behavioral sciences early in idea generation and study design is imperative for the successful conduct of biomedical trials and for ensuring optimal data collection approaches necessary for the interpretation of findings, particularly in cases of unexpected results.